Underdeveloped chick

I have 6 chicks who are 4 days old. 5 of them are developing fine. One I notice is smaller and her wings aren't as developed as the rest of my small flock. I called tractor supply and the woman told me to add some electrolytes to their water and to give that chick some chicken grit because she is also the only one to continue to have pasty butt. They told me I could bring her in and swap her out for a new chick, but I can't give up on her. She's doing better now that I have added some electrolytes to their water. I'm just concerned why she's under developed. I gave them.all a small amount of chicken grit in my hand. Then I changed out their food with a fresh batch. She's eating and drinking fine. I'm just worried that something is wrong with her. Before the electrolytes she separated herself and slept often. Seems to be doing a bit better. Is it normal? Will her wings finally grow out? And advice or support would be nice. I'd be pretty sad to lose and of them. They're so awesome!